1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a pilot designing method for channel estimation on the uplink in an orthogonal frequency division multiple access (OFDMA) system.
2. Description of the Related Art
High-speed, high-quality data transmission is required for next-generation mobile communications to provide a variety of multimedia services with improved quality.
Orthogonal Frequency Division Multiplexing (OFDM), on which OFDMA is based, boasts high-speed communication due to low equalization complexity for frequency-selective fading channels. Thus, OFDM is widely used as a physical layer transmission scheme in various wireless communication systems including wireless local access network (WLAN), digital TV broadcasting, and next-generation mobile communication systems.
OFDMA is a multiple access scheme of allocating different subcarriers to a plurality of users. On the downlink, all users can use pilot information for channel estimation, which facilitates pilot transmission and channel estimation. On the uplink, however, when a particular use transmits pilot information in a predetermined pilot sample, other users are prohibited from using the same pilot sample. Therefore, many users share a fixed number of pilot samples. The limit of the pilot information which is available to each user degrades channel estimation performance.
FIGS. 1A and 1B are graphs which illustrate subcarrier allocation to users in a known uplink OFDMA system.
Referring to FIGS. 1A and 1B, the uplink OFDMA system allocates resources to users, each on the basis of a transport block 110 within the same frame. The transport block 110 comprises data subcarriers 103 and pilot subcarriers 105. Only a pilot channel in the transport block 110 is used for channel estimation of the user allocated to the transport block 110.
FIG. 2 is a channel gain graph illustrating interpolation-based channel estimation in the uplink OFDMA system. When user #1 and user #2 are allocated to resources within the same frame, pilot subcarriers 205a of user #1 and pilot subcarriers 205b of user #2 are arranged equidistantly on the frequency domain. The channel between the pilot subcarriers 205a (or 205b) is estimated by interpolating the channel estimates of pilots within the transport block of user #1 (or user #2). However, at the boundary A between the transport blocks of the two users, a pilot subcarrier is nonexistent for user #1 on the outside of his transport block and thus a reference channel estimate is not available for interpolation. As a result, a channel estimation error increases, which leads to the degradation of the whole data detection performance. To solve this problem, it has been proposed that an additional pilot subcarrier be allocated at the boundary A, or that a pilot subcarrier be disposed at the boundary without increasing the number of pilot subcarriers. Yet, the additional pilot subcarrier allocation decreases band efficiency, and the arrangement of pilot subcarriers at wider intervals without increasing the number of the pilot subcarriers increases a channel variation between pilot subcarriers, thereby increasing a channel estimation error.